In computer graphics, the device that displays the desired graphic output, typically a special-purpose cathode ray tube (CRT), may either be an analogue-driven device or a digitally-driven device.
In a digitally-driven device, an individual vector is drawn by first brightening-up the electron beam to display a single dot then, after some suitable delay, blanking the beam to cause the dot to disappear. Next, the deflexion potentials applied to the X and Y deflexion circuits are incrementally increased to move the beam through some discrete displacement in a direction lying along the desired vector. The beam is then brightened-up again to display a second dot. This process is repeated until the desired vector has been drawn on the face of the CRT.
A close examination of the display screen will reveal that the vector is actually comprised of a series of closely-spaced dots; however, at normal viewing distances, these dots tend to merge giving the impression of a continuous, illuminated line, i.e., the desired vector.
In an analogue-driven device, on the other hand, the same vector is drawn by first brightening-up the beam, then continuously increasing the X and Y deflexion potentials to sweep the fully brightened-up beam in the desired direction until the end point of the vector is reached, at which time the beam is blanked-off.
Digitally-driven display systems have several advantages not possessed by analogue-driven systems. Among these are the fact that: (1) the deflexion logic is simple to implement; and (2) accurate digital information concerning the instantaneous position of the electron beam is always available. This latter fact is particularly advantageous if a light pen is associated with the graphics display. Furthermore, in a digitally-driven system, it is easy to detect edge violations because the moment that the beam goes off the edge of the screen an overflow condition must, of necessity, exist in at least one of the registers which keep track of the beam position.
Notwithstanding the above, there are some serious drawbacks to the use of digitally-deflected display devices. Among these is the fact that because the beam is moved in a series of discrete steps, rather than continuously, the display can only be brightened-up after the beam has settled down to its steady-state position. However, as is well known, the digital-to-analogue converters (DAC's) which are typically used to convert the digital beam-position information into the analogue signals actually required to deflect the beam are far from perfect, and it takes a small but finite time for the analogue output to catch up to the digital input. This effect is particularly troublesome when there is a change in one of the more significant bits of the input to the DAC. In the case of the most significant bit (MSB), for example, a change from a binary "0" to a binary "1" represents but a single increment in beam position but, nevertheless, cause a major change in the format of the bit pattern comprising the digital word. This, in turn, results in an unwanted and troublesome transient in the output of the DAC. This transient is caused by the one-bit change in the MSB rippling-through all of the stages in the DAC. Because of this problem, a typical digitally-deflected graphics device is forced to use a relatively long time intervals between successive deflexion points.
For these and other reasons, considerable interest has been expressed in a return to analogue deflexion even though, heretofore, analogue deflexion apparatus has been unable to furnish accurate information concerning the instantaneous position of the electron beam and lacks the ability to quickly detect edge violations.